1) Field of the Invention
The present invention relates to a spark plug for an internal combustion engine which ignites an air-fuel mixture in a combustion chamber, and more particularly to a multi-point spark plug including a plurality of center electrodes.
2) Description of the Related Art
For example, as disclosed in Japanese Patent Laid-Open No. SHO 57-208084, there has been known an ignition device for an internal combustion engine having a center electrode and a third electrode disposed in parallel with the center electrode.
In addition, as shown in FIG. 20, there has been designed a multi-point spark plug 100 having a pair of center electrodes. This multi-point spark plug is made up of an insulator 104 internally including two center electrodes 102 and 103 in a parallel configuration, a housing 105 for holding the insulator 104 and two earth electrodes 106 and 107 located at one end portion of the housing 105 and disposed in opposed relation to the tip surfaces of the center electrodes 102 and 103 in a state where a discharging gap is interposed therebetween. Moreover, two terminals 121 and 122, respectively connected electrically to the two center electrodes 102 and 103, are put in two axial holes 111 and 112 made in the insulator 104, respectively.
There is a problem which arises with the above-mentioned multi-point spark plug 100, however, in that, since the two terminals 121 and 122 are respectively placed in the two axial holes 111 and 112 made in the insulator 104, difficulty is encountered in making a connection easily to a plug cap. This will be described hereinbelow with reference to FIG. 21.
FIG. 21 is an illustration of a plug cap 130 to be connected to the multi-point spark plug 100 shown in FIG. 20. This plug cap 130 is composed of a blind-end cylindrical cap portion 131 to be set to cover up (cap) a head portion 114 of the multi-point spark plug 100, and an ignition coil portion 132 for generating a voltage to be applied to the multi-point spark plug 100.
A first contact terminal 133 and a second contact terminal 134, each of which comprises a spring provided at a tip surface of a bar-like base, protrude from a bottom surface of the cap portion 131, and the first and second contact terminals 133 and 134 are brought into contact with the first and second terminals 121 and 122 of the multi-point spark plug 100, respectively, thereby supplying a voltage to the multi-point spark plug 100.
The ignition coil portion 132 has a box-like casing for internally accommodating an ignition coil, and a fitting hole 139 is made in one side surface of the casing 138 so as to permit the insertion of a bolt thereinto.
Secondly, a description will be given hereinbelow of the attachment (connection) of the plug cap 130 to the multi-point spark plug 100. The multi-point spark plug 100 is fixedly secured to an engine in a manner such that a male screw 108 formed in the housing 105 is put into a threaded engagement with a female screw formed in a cylinder head (not shown). The plug cap 130 is set to cover the head portion 114 of the multi-point spark plug 100. At this time, the first contact terminal 133 and the second contact terminal 134 are inserted into the axial holes 111 and 112, respectively. Following this, a bolt (not shown) is inserted into the fitting hole 139 attached to the casing 138, and this bolt is placed into a threaded engagement with a female screw set on the cylinder head (not shown), thus fixedly securing the plug cap 130 thereto.
However, in the case of this multi-point spark plug 100, for setting the plug cap 130, there is a need to insert the first contact terminal 133 and the second contact terminal 134 into the axial holes 111 and 112. Therefore, it needs that, after the plug cap 130 and the multi-point spark plug 100 are disposed so that their axes align with each other, the plug cap 130 is rotated around its axis to align the first contact terminal 133 with the axial hole 111 and further to align the second contact terminal 134 with the axial hole 112 and the plug cap 13 is pushed into the multi-point spark plug 100 to be fitted thereto. This makes it difficult to accomplish the attachment of the plug cap 130 easily.
In addition, since the attachment of the multi-point spark plug 100 is made in a manner such that the male screw 108 formed in the housing 105 and the female screw formed in the cylinder head are placed into engagement with each other, the positions of the axial holes 111 and 112 do not settle when the multi-point spark plug 100 is fixedly secured to the cylinder head. This is because, when two members are fixedly coupled to each other through screws, the relative positions of the male screw side member and the female screw side member in rotating directions vary according to the tightening torque and, even if the tightening torque is controlled to stand at a constant value, the precise screw processing is a necessity to making the rotating-direction relative positions constant. Thus, in a case in which the positions of the axial holes 111 and 112 do not settle, the position of the fitting hole 139 attached to the plug cap 130 engaging with the axial holes 111 and 112 does not settle. For this reason, the fitting hole 139 does not necessarily align with the position of the female screw formed in the cylinder head and, hence, difficulty is experienced in fixedly securing the plug cap 130 through the use of a bolt.